A lithium ion secondary battery including a positive electrode and a negative electrode that occlude and release lithium (Li) and a non-aqueous electrolyte has a high voltage and a high energy density. Hence, the lithium ion secondary battery is drawing attention these days as a power source of mobile phones, notebook personal computers, electric tools, electric automobiles, etc. However, in order to use the lithium ion secondary battery as power sources of wide-ranging applications, further improvement in energy density and higher input characteristics than existing ones are desired. The improvement in input characteristics is important not only to improve user's convenience but also to suppress the increase in the weight and size of the battery.
In the lithium ion secondary battery, the upper limit of the charging rate is influenced by the charging acceptability of the negative electrode, and the charging acceptability is greatly constrained not only by the performance of the negative electrode active material but also by the ion conductivity of the electrolyte. For example, when the non-aqueous electrolyte of the lithium ion secondary battery and an aqueous electrolyte of a nickel-metal hydride battery are compared, the ionic conductivity of the former is approximately one to two orders lower than the ionic conductivity of the latter. The difference in ion conductivity appears in differences in electrode thickness and charging rate between the lithium ion secondary battery and the nickel-metal hydride battery.
For the ion conductivity of the electrolyte, not only the ion conductivity of a separator layer (or an electrolyte layer) between the positive and negative electrodes but also the ion conductivity in the positive and negative electrodes is important. In the lithium ion secondary battery, when an electrolyte with a low Li ion conductivity is used, the overvoltage in the electrode during charging and discharging will become large, and particularly when the charging rate is high, the electric potential of the negative electrode will become a minus and this will cause deposition of Li metal and a short circuit between the positive and negative electrodes.
To address such a problem, the following technologies are proposed.
In Patent Literature 1, a cation conductive medium in which an inorganic oxide with a specific surface area of 0.3 to 50 m2/g is put in an ionic liquid to improve the cation conductivity in the ionic liquid is proposed.
In Patent Literature 2, a negative electrode for a non-aqueous electrolyte secondary battery in which a nanoparticle-size ceramic is dispersed in a negative electrode to achieve high input/output density and cycle characteristics is proposed.